Modulator



June 26,1945. R, c, SHAW 2,379,042

MODULATOR Filed April 28, 1942 HIGH 1 Fasaunvcr V fl LOAD xc/r4 TION F t &

. E uoauumvc SIGNAL 55 H. E GENERATOR uooumrwa I T SIGNAL 5 s cave/arm v 1 1 MODULAT/NG GIVAL ATTO/MEY Patented June26, 1945 MODULATOR Robert 0. Shaw, Holmdel; N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corp oration of New York Application April 28, 1942, Serial No. 440,811 I I 3 Claims.

This invention relates to systems of modulation involving amplifiers operating with low distortion.

One object of this invention is concerned with a system of modulation, involving feedback am- Dliflers and operating by variation of the feedback in accordance with modulation potentials applied within the feedback loop;

A further object of this system of modulation is to vary the effective loss in the 5 (beta) circuit of a low distortion negative feedback carrier amplifier in accordance with modulating potentials, applied to the grid of a variable gain vacuum tube located in the ,B circuit.

Referring to the figures of the drawing:

Fig. 1 is a schematic diagram intended to 11- lustrate the basic principles of the invention; and Fig. 2 is one practical embodiment'of the novel modulator wherein a vacuum tube device is utilized to control the effective loss in the feedback loop of the negative feedback carrier amplifier.

Referring to the, schematic circuit of Fig. 1 specifically, a carrier frequency amplifier utilizing stabilized negative feedback is shown, with a forwardly feeding or a path, and a feedback or p path, in combination with means for varying the amount of negative feedback in accordv ance with modulating signal potentials.

The a section of the negative feedback amplifier is conventional in design, with normal excitation from a source of wave energyto be moduwhere I E=output voltage Ec=carrier input voltage =complex amplificationfactor I in the forward p=complex loss factor in the feedback path and it can be assumed that Now; if I5] were to be varied by means of the modulating voltage, the output voltage E wouldwhere k is a constant correlating the change in 5 the lated and with normal loadconnections. The p circuit is modified to provide an effective loss whose amount can be varied in accordance with the amplitude of a modulating voltage.

' Essentially, the principle of operation of the system involves the amplification of a carrier wave in a. low-distortion stabilized negative feedback amplifier and the variation of carrier feedback in accordance with a modulating potential applied within the feedback loop to produce a modulated wave in the output load.

modulating signal; a; is the loss factor for no modulation, (1. e.. when k or E's-=0) E8 is the peak modulating signal amplitude having the frequency Over one cycle of the envelope frequency, ,9

. will vary between the limits of $0 (likEs). The

intermediate values will depend on' the value of cos st.

The mathematical and theoretical relation ships applying to the schematic of Fig. 1 are briefly derived from the following considerations: Assuming that the input and output circuits of the feedback amplifier are conjugate to the p-feedback path, then the following relationships exist:

- J -(m 1tfl -fi -111 (I) If the carrier wave is Ec cos ct having peak amplitude of E0 and frequency the output voltage will now be 8. frequency signal bland carrier wave having a degree of modulat on k.

The following table illustrates this modulation process under the conditions set forth above and as on the operating points selected. It may be desirable in some applications to make It a nonlinear function by altering the characteristics of the amplifier t: by appropriate means.

been omitted from Fig. 2 for purposes of simplification.

Fig. 2 shows a carrier frequency feedback amplifier driven at frequency by the high frequency generator I and supplying power to a load such as an antenna 2. The amplifier A has a a path and a 8 path. The 1* path, which is the forwardly feeding portion of the amplifier, is in conventional arrangement. However, in-the dcircuit an amplifier tube i1 is inserted suitable for amplifying at frequency The tube t1 is of the variable mu type so that its gain and thus the effective lossin the p circuit can be varied by means of its grid bias. The grid bias is provided by a direct current electromotive force in series with the modulating electromotive force E, supplied by the amplifier t2 operating at and fed from a modulating signal generator 2.

Thus, voltage variations in the modulating produce corresponding changes in loss in the feed-. back amplifier p circuit. The factor k of Equation 2, as it applies to the circuits of Figs. 1 and 2 70 factor-B.-

dependsonthetubeandcircuiteiementsaawell where flo has been arbitrarily chosen as ,4,0 [0, 5 Transmission through the B circuit is deterkE==0.67 and =-1,000 l0. mined by the coupling circuits, tube ti and the Amount o t t u di Common fl (l+kE. cos at) I 101 582312 bifc k ll-pfil Decibels No modulation cos =0... 30 30 E. cos ct 30.44 0.972 Medullation trough cos 18 18 E. cos ct. 35.05 0.982

a Modullation peak cos 90- 90 E, cos ct 21.65 0.918

It may be seen that a variation in the amount direct current and signal frequency biasing elecof feedback from approximately 22 to 35 decibels tromotive force applied to the control grid.

resulted in the carrier being modulated by 67 per What is claimed is: cent. The deviation in linearity of the output en- 1. A method of modulating a high frequency velope from the modulating signal, assuming carrier wave of frequency |m8| 1 can be determined by the departure of c I '25 5; 1 with a signal wave of frequency from 1. It is shown in the table that the devia- 1 Y tion, for this example, amounts to only a few per cent and with the distortion correspondingly small. said method comprising amplifying the carrier Representative embodiments of modulation cirvoltage between an input and an output point. cults utilizing the principle of modulating in the negatively feeding back therebetween a portion ,6 loop of a carrier feedback amplifier are shown thereof under the operating condition I fil l in Fig. 2. Power supply showings, and other conwhere ventional accessory and by-pass circuits, have ;t=complex amplification factor in the forward feed path B=loss factor in feedback loop and varying said transmission loss factor 5 as a function of the modulating signal voltage.

40 2. A modulation system comprising a source of B=the loss factor in the negative feedback path,

high frequency carrier oscillations, at low distortion negative feedback amplifier for amplifying said high frequencies, and adapted to operate uriderthe condition .c}3| 1 where =thecomplex amplification factor in the forward feed path,

condition where E=the output voltage of the amplifier,

Ee=the high frequency input voltage. B=the feedback ton loss factor,

and means for modulating said high frequency oscillations by varying said ion loss ROBERT C. SHAW. 

